In agriculture, satellite-based position-finding systems are used, inter alia, for automatic steering of agricultural vehicles, for position-specific recording of data, for example yield data, and for position-specific driving of actuators, for example for the spreading of fertilizers or chemicals.
Satellite-based position-finding systems such as GPS, Glonass or the future Galileo use a plurality of satellites which are in orbit around the earth, are equipped with atomic clocks and in each case transmit electromagnetic (radio) waves which contain time and identity and/or position information. The associated receivers each have to receive the signals from at least three satellites in order to allow them to determine their current position in three dimensions. If, in addition, it is also intended to determine the current time, it is necessary to receive signals from four satellites.
It is also normal practice (see EP 0 660 660 B) to improve the accuracy of position-finding systems by reception of radio signals which contain correction data and are transmitted from reference stations at known locations. These systems are referred to as differential position-finding systems (for example, DGPS). The reference stations receive signals from satellites in the position-finding system and use these signals to derive position information, which they convert to correction data and transmit by means of a radio transmitter to the vehicle, which is equipped with a suitable receiver for reception of the correction data. A computer uses the correction data and the signals received from the satellites to determine the position of the vehicle. On the one hand, so-called wide area systems with stationary and networked reference stations, and on the other hand local reference stations are known, which may be mobile and are in general positioned in the vicinity of the respective field to be worked on, or are installed permanently at elevated locations. One advantage of the use of a local reference station for position finding opposed to a wide area system is the considerably greater accuracy which is achieved because network delay times are avoided, and the correction signal is generated in the immediate vicinity of where it is being used. Systems such as these are used for dynamic measures or for steering of moving machines, because of their high accuracy, and are thus also referred to as Real Time Kinematic (RTK) systems.
Mobile reference stations are subject to a certain risk of being moved during operation, for example because they are blocking a track or the like and are therefore moved out of the way by a third party. A reference station installed on a stand can also be blown over by a relatively strong wind. Any movement of the reference station leads to an analogous movement of a vehicle which has been automatically steered on the field, and thus represents a considerable safety risk.
This is because the reference station cannot itself decide whether the position change is caused by movement of the station itself or leads to the supposition of other errors or faults. In fact, it still transmits the correction data to the vehicle, although this is now based on a new position of the reference station. Since the vehicle computer is now presented with different correction data, it also calculates different position information. Since this differs from the nominal position, a vehicle steering system produces correspondingly counteracting steering data which leads to the undesirable steering movement that has been mentioned, in order to guide the vehicle back onto the nominal path. Only if a position change assumes values which lead to the belief that system-imminent errors or faults are improbable will the reference station transmit a warning message to the vehicle, and will switch the correction signal off. However, by this time the vehicle may already be several meters away from the nominal course.